Facsimile system



K. W. PFLEGER FACSIMILE SYSTEM Aug. 26, 195.2l

4 Sheng-sheet 1" Filed Nov. 15, 1946 t :It .S

OEE i Y' ik INVENTOR K W PFLEGER ATTORNEY fl .Sheets-Sheet 3 K. W. PFLEGER FACSIMILE SYSTEM blanks Aug. 26, 1952 Filed Nov. 15,

Aug- 26,1952 K. PFLEGER 2,608,616

I FACSIMILE SYSTEM Fil-ed Nov. 13, 194e I 4 Sheets-Sheet 4 F/G.8 V

A W//A W//M WA] I, A A A A I2 A L /N VE N TOR K. w. PFLEGE/f? A TTOR/VEV Patented Aug. 26, 1952 UNITED STATES PATsNToFFIcE FACSIMILE SYSTEM Kenneth W. Pfleger, Arlington, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application November 13, 1946, serial No. 709,576

24 Claims. 1 t

This invention relates to signaling and particularly to apparatus for facsimile transmission.

When printed, typewritten, or handwritten matter, drawings, or other two-tone subjectmatter or copy is transmitted from point to point by means of atelephotograph system of the usual type set up for half tone operation, full use is not made oi the ability of the system to transmit reliably a relatively large range of signal amplitude values. Nor is advantage taken of certain 'characteristics of ordinary two-tone subjectmatter to effect economies in the number of signal elements required to be transmitted for faithful reproduction. Reference is here made, first to the fact that, in all two-tone subject-matter, theV copy areas encountered by a scanning element traveling along any path may be regarded asoccurring in pairs, a light area following each d a-rk area, and vice versa; and, second, to the fact that areas of one character, for example, the light areas, usually far outnumber the areas of the other character, for example, the dark areas.

An object of this invention is to increase the transmission rate of two-tone subject-matter or copy over any channel by utilizing the special characteristics of such subject-matter to the best advantage.

Anotherobject is to increase the transmission rate of two-tone subject-matter or copy over an ordinary transmission channel by effecting a more equal distribution between band width requirements and amplitude discrimination requirements than is customary whereby the rate of transmission over a channel capable of handling a range of amplitude greater than necessary to transmit signals derived from the scanning of such copy by conventional methods may be increased.

In accordance with one feature of the invention the signaling speed or transmission rate is increased by analyzing the copy in a fashion to produce conventional image signals and to derive therefrom an intermediate sequence of signalelements each of which corresponds, not to a dark area nor toa light area nor to the transition between them, but to an adjacent pair of` areas of unrestricted length, of which one is a light area and the other is a dark area. This results in a reduction to substantially one half of its conventional value the number of different signal elements which must be transmitted in a given time for faithful reproduction of the copy, even with copy which is, on the average, one half light and one half dark.

It is necessary, however, to distinguish,in the copy analysis, between the different kinds of area pairs, and in transmission and reproduction between the signal elements which represent the different kinds of pairs. Otherwise the reproduction would merely be of alternating light and dark areas of like configuration. Thus, for example, a pair consisting of a short dark area followed by a short light area must be distinguished from a pair consisting of a short dark area followed by a long light area.

In accordance with a further feature of the invention this distinction is effected by establishing a relation between the character of the copy area pair, i. e., its long dark-short light or long light-short dark etc., character, and some suitable characteristic of the signal element which represents it. Thus the length of one member of the area pair, for example the dark member, may be related to the amplitude o-f the signal element which represents the pair. Still further flexibility is achieved in one embodiment, by causing successive signal elements to -alternatein polarity, as well as differing in magnitude.

The utilization of differences in magnitude and in sign or polarity of signals, to carry information, in addition todiierences in duration, is well known in the telegraph art. Cardwell 'Patent 801,967 and Deardori Patent 1,614,546, among others, contain such teachings. Various proposals have also been made to derive signaling economies from the characteristics of particular types of transmissible copy. For example, Gray Patent 2,138,577 teaches the transmission ofsignals related to the transitions or discontinuities between dark areas and light areas, instead of signals related to these areas themselves, and Watson et al. Patent 1,602,469 teaches the use of a dual signal having a primary component representative of the occurrence of a picture element and an auxiliary component indicative of the length or spatial extent of the picture element and controlling the duration of the primary component. Again, various means have been proposed for effecting economies by temporary signal storage prior to transmission. Baird Patent 1,945,626 is representative of a large classlof such suggestions.

The present invention differs' from the suggestions of the prior art in various .features and relations. Among these dilerences are the `novel relations which `it establishes between copy areas and signal elements, which relations offer marked.` advantages in the increase of signaling speed.`

The invention differs also in the organization of the apparatus elements employed for establish- 3 ing these relations at the transmitter and for reconstructing the copy, at the receiver, from signals in which these relations have been established.

The invention will be more fully understood from the following detailed description of preferred embodiments thereof taken in conjunction with the. appended drawings, in which:

Fig. 1 is a schematic diagram of analyzer and transmitter apparatus in a facsimile transmission system in accordance with the invention;

Fig. 2 is a schematic diagram of receiver apparatus adapted to reconstitute copy analyzed and transmitted by the apparatus of Fig. 1;

Fig. 3 is a schematic diagram of a modification of a part of the apparatus of Fig. l, to the right of the lines X-X;

Fig. 4 is a schematic diagram of a modification of a part of the apparatus of Fig. 2, to the left of the lines Y--Y;

Fig. 5is a diagram showing the development of a signal by the apparatus of Fig. l into special form and the reconstruction therefrom of a copy signal by the apparatus of Fig'. 2;

Fig. 6V is a schematic diagram of analyzer and transmitter apparatus organized in accordance with another embodiment of the invention;

Fig, 7 is a schematic diagram of receiver apparatus adapted to reconstitute copy analyzed and transmitted by the apparatus of Fig. 6; and

Fig. 8 is a diagram showing the development of a signal by the apparatus of Fig. 6 into special form for economical transmission and the reconstitution therefrom of a copy signal by the apparatus of Fig. 7.

f Referring now to Fig. 1, the left-hand portion of `this gure shows, in highly simplified form, a'system for deriving a sequence of conventional image signals from copy to be transmitted, i. e., signals which are directly proportional in magnitude and duration to the light and dark areas of the'copy. The'copy I to be transmitted, which is preferably of such a kind that the lengths ofy the darkV areas along4 any scanning line are equal or nearly equal, may be wrapped around a drum i and held in place by conventional means-such as spring clips, not shown. The drum 2 is arranged for rotation about its axis and for advancement parallel with its axis, either continuous' or intermittent, an arrangement for continuous advancement being shown in the drawingbecause of its simplicity. Thus the drum 2 is mounted lon a coaxial mandrel,V one extended end- Sjoff which is threaded. The threads engage with a threaded bushing 4. The other end Eijofv the mandrel turns freely in another bushing I `Any suitable means, here schematically indicated by a pulley 1, driven by a motor 8, may be employed for simultaneously driving the drum in rotation and in axial advancement.

A small region of the drum is brightly illuminated as by an aperture 9 in a screen I floodlighted by a lamp II, which aperture is focussed onto the surface of the drum by a lens I2. This illuminated part of the drum is in turn focussed by a second lens I3 onto 'the light sensitive portion of a photoelectric cell I4. As the drum, bearing the copy to be transmitted, revolves, the illuminated area scans the copy'I and the photoelectric cell I4 thus Vreceives lightVr which isV pro'- portional to the light anddark value or ldensity of that portion of the copy I which is instantaneously illuminated. In accordance with known principles, the photoelectric cellY I4 then delivers an' electric signal of conventional form, directly 4 proportional to the instantaneous light values of the copy. This signal may now be inverted, in known fashion, to provide a signal whose amplitudes are directly proportional to the copy density; i. e., maximum signal amplitude corresponds to black copy. Referring to Fig. 5, curve A representsV the light and dark values of adjacent areas of the copy taken along a particular scanning line, and curve B represents the resulting inverted photoelectric cell signal. This signal may be in an actual case of less perfect definition than is indicated by curve B. As long as the distortion is not too severe, it is of no importance. If it should be excessive, a shaping circuit of a type well known in the art may be employed to restore it, with or without a small and unimportant time delay, substantially to the form indicated by curve B.

This signal may now be brought to a suitable power level as by an amplifier I5, which may also include the inversion feature above referred to, and applied to the analyzing apparatus of the invention.

In accordance with a first modification of the invention the signal is first fed through a socalled phase sp1itting'circuit I6 which derives from it two different signals (curve C and curve D of Fig. 5), and sends them out on two different paths or lines, indicated in Fig. l as phase A and phase B. In the phase A line, the outgoing signal (curve C) commences at the rst positive transition of the curve B signal and terminates at the'next positive transition. YIn the phase B line, the outgoing signal (curve D) commences at the first negative transition of the curve'B signal and terminates at the next nega-- circuit arrangement is a network of polarizedv relays, each of which is so designed that its movable contact remains on whichever one of its two fixed contacts it is resting when itsrwinding' is deenergized. These relays, when interconnected in the manner described in the aforesaid Nyquist patent, produce the following sequence of operations, when the circuit arrangement is,

energized by the curve B signal, i. e., by the amplied and inverted output from the photoelectric cell:

First dark area-energizes phase A. Light area thereafter-energizes phase B. Second dark area--deenergizes phase A. Light area tliereafter-deenergizes phase B'.

Thus two cycles of the photoelectric cell current, curve B, produce only one cycle of the signal 'current in each of phases A and B, .which constitute the output of the phase splitting` network. Examination of curves C and D of Fig; 5

will reveal Ithat the signals on phases A and BV are characterized in the following ways: First, each signal on phase A commences with a dark area of the scanned copy, while each signalen phase B commences with the following light area.

econd, they differ in duration only to the comparatively slight extent that one copy area pair. commencing `with a dark` area, differs from another copy pair which commences with a light area. Third, the signals of phase B commence later than those of phase A by a time corresponding, for a given scanning speed, with the length measured along the scanning path, of a single dark copy area. Thus curve D of Fig. 5 is displaced, on` the drawing, to. the right of curveC by substantially the width of one dark signal of curve B.

In accordance with the invention, the phase A signalsare next delayed with respect to the phase B signals by a time equal to their original time displacement, so as to bring them into substantial'coincidence. This signal delay may be accomplished by the Vuse of apparatus ofV any desired type, schematically indicated in the drawing by the block I1. Evidently the current in the A phase at the output terminals of this device may now be represented by the curve C' of Fig. 5.

In accordance with the invention, the delayed A phase signals and the undelayed B phase signals'are combined to form a compositesignal, suitable for economical transmission. The combination may be eifectedin various ways. One way is to supply the two signals to be combined to two relays, one I8 of which alters only the magnitude of an outgoing line signal while the other I9 alters only its polarity. (For the present the switch Si, to be described below, is to be considered closed.) sorted to, to accomplish this result, a simple one being shown in the drawing wherein a voltage source such as abattery 20, supplies aivoltage of six units to a potentiometer 2I whose mid-point 22 is grounded, taps being connected to the potentiometer at the points whose voltages, measured from ground or zero, are, respectively, 3E, -E, `-1-E and |3E. The A phase relay I8 has four xed contacts and two movable 'tongues which' are mechanically arranged for synchro'- nous movement, the vfixed contacts being connected in the manner shown to the potentiometer taps. The movable tongues are connected to the upper and lower fixed contacts. respectively, of the B phase relay I8 which may bea simple device having two fixed contacts and one movable tongue, which latter may be connected to one terminal of a transmission line 23 whose other terminal is connected to ground.

With the tongues of both relays I8, I9 in the positions shown, these relays are deenergized, and the outgoing signal :voltage applied to the transmission line 23 is -E. If the B phase relay I9 were alone energized, its tongue `would move to the opposite fixedr Contact and the line voltage would be changed to +E. If instead the A phase relay IilA were alone energized, both of its tongues would move to` their opposite contacts, the upper one alone lof the lower pair of contacts being eiective, and the line voltage would be 3E. If both of the relays were energized together, all contacts would move upward and the line voltage would be +3E. A

As above stated the A phase signals have been delayed with respect to the B phase signals by an amount such that, for the most part, their transitions occur together. Thus the signals which are applied to the transmission line are depicted in the curve E of Fig. 5. They consist, for the most part, of either +3E or -E, the remaining amplitude .values of -l-E and 3E ocourringonly .on occasions vwhen vthe copy I de- `Various means `may be reparts from the preferred form in which `al1 dark areas are of equal length, no restriction being placed on the length of any light area. Such a. condition appears in the second signal group of. curve E, and is due to the fact that the fourth dark area of curve A is longer than the others. Thus, onthe average, the number of transitions between signal elements of the intermediate signal sequence on the transmission line has been substantially reduced. Even if the transmission line 23 should fail to transmit correctly the signals of amplitude values -I-E and -3E, the results would be merely a degradation of the quality of the reproduced copy which would generally be tolerable and not render it illegible. However, for optimum results, it is recommended that the copy employed be of the preferred type. in which every dark area is of Substantially the same length as every other dark area.

Another system by which the delayed A phase signals may be combined with the undelayed B phase signals to form composite signals suitable for transmission in accordance with the invention operates by simple arithmetical addition. This result may be secured by means of adding circuits of various sorts, the one shown in Fig. 3 being representative. Here the delayed A phase signal actuates one relay I8' and the undelayed B phase signal actuates another relay I9. Each relay comprises a single fixed contact and a movable tongue. The fixed contact of one relay I8 is connected to a source of substantially constant current, for example, a source of electromotive force 24 in series with a comparatively high resistor 25. The xed contact of the other relay I9 is similarly connected to a constant current source comprising a source of electromotive force 26 and a high resistor 21. The two movable tongues are connected together and to one side of the outgoing transmission line 23. The other side of the transmission line 23 may be connected to ground.

In operation, the `switch S2 being closed, a signal on the B phase operates the relay I9 and current of aspecied value, for example, unit current, enters the transmission line 23. Similarly a signal on .the A phase operates the other relay I8' which likewise delivers unit current to the transmission line 23. When there is no signal on either the A phase or on the B phase, neither relay is operated and zero current is -delivered to the transmission line. When signals appear on both the A phase and the B phase simultaneously, both relays are operated simultaneously and two units of current are delivered to the transmission line.

taneous presence of impulses in both phases, graphically represented by the coexistence of current in the C curve and the D curve., gives the largest composite signal, for example, two current units, and the absence of both gives the smallest, for example zero current. copy of the preferred form, the out-going line signal has at all times one or other of two easily distinguishable Values, while the intermediate signal values occur only when the copy departs from this preferred form.` Curve E of Fig. 5 rep resents this condition if the indicated current values +3 and 1, as well as the` zero current axis, be ignored.

The intermediate signal sequence (curve E) which is thus generated is now transmitted to receiver apparatus located at a distant point by any suitable transmission channel, schematically When this arrangement is employed, the simul-Y Thus with indicated on'FigS. 1 and 3 by the conductors 23 and23respectively.

Atthe receiver station, the signal as received may be degraded in the Vsense that its transitions are less*` sharp than the transitions of curve E', this degradation taking place by reason of the characteristics of the transmission line. and associated terminal apparatus. The signal may have characteristics .approximately as indicated in curve For Fig. 5. Despite certain degradation of the'sharpness or its transitions, it nevertheless retains the fundamental character of the signal of curve E'.-

VIf desired, the degradation may be removed by any suitable type of signal shaping circuit. Many and various circuit arrangements and devices for squaring up the transitions of transmitted signals are known in the arti Patent 1,315,539 to J. R. Carson. Patent 2,202,629 to C'. W. Hansell and P atent 2,345,881 to Harry Nyquist are representative of various types of such arrangements. Even though the degradation due to transmission over the channel be considerably greater than that indicated by the change from curve E to curve F, the use of such a shaping circuit will restore the signal substantially to the form of curvev E, slightly delayed however. The use of such a shaping circuit is, however, only a rennement, and with degradation no more serious than that-indicated, such renement is unnecessary.

Turning now to the receiver apparatus indicated in Fig. 2, the received intermediateV signal sequence (curve E or F of 5, depending upon whether or not this signal has been degraded without being reshaped),v appears at. the receiver terminals 30 of the transmission line. It may be amplified or modified in character as desired,v by the use of terminal apparatus 3 I. It is then appliedto two translationv devices such as relays 32, 33., reach of -which has two fixed contacts and one movable tongue. The relays may be alike except'for the fact that movement of the tongue of one relay 33 is delayed Iwith respect to movement of the other tongue by any desired means, for example, the tongue of the slow-acting relay 33 may be mechanically linked to a paddle 34 which moves in an oil bath; i. e., it may be coupled with a dash-pot 35. The lower contacts of these two relays are connected to ground, and

the upper contact of each relay is connected to the positive terminal of a battery 36, 31 whose negative terminal is connected to ground. The tongues' of these two relays are connected by way of a rectifier 38, a pulse Shaper 39 and an amplifier '40 to a current controlled light source, schematically indicated by a lamp di. The light of the: lamps M, when illuminated, is focussed by a lens 42 onto a small element of the surface of a reproducer drum 2 which may be identical with the drum 2 lat the transmitter and may be similarly moved both rotationally and axially, this movement being preferably suitably coordinated with that at the transmitter by synchronizing circuits or devices which are not shown but lwhich are well known in the art. Parts of this receiver apparatus which correspond with similar transmitter parts are indicated by like reference numerals, distinguished by primes.

l In operation, as the signal F V(or E) `arrives at the receiver apparatus, both of the relays 32 and 33 are energized simultaneously. The upper relay 32 operates substantially instantaneously and the lower relay 33 having not yet operated, the voltage E of the battery 36 is applied to the rectiiier 38 which -delivers a signal which after '8i amplification and shaping, if necessary, energizes the. lamp 4l which brightly illuminates .a :small area. of the rotating receiver drum 2'; The drum mayhave wrapped. around it anunexposed sheet of photographically sensitive paper l or ,nlm which is exposedat a particular part of its surface by the light fromV the'lamp 4I. After "a briei interval, however, theftongue of the lower relay 33 movesupward and places the same voltage E on the lower terminal of the rectier 38.` This reduces the voltage input to the rectifier to zero, and the lamp 4l is extinguished. This takes place after a delay determined by the design and adjustment of the. dash-pot 35 which is preferably made substantially equal to the time required to scan across a single dark copy area.

At the next transition of the signal F '(or El i. e., atthe Iirst negative transition, the relays are bothA deenergized and the tongue of the upper relay 32 moves to itsl lower contact. The'tongue of the lower relay 33 remains against the upper contact until the expiration of the delay period determined by the setting ofthe dash-pot 35, whereupon the lower relay tongue moves to its lowerxed Contact. During the interim of the delay a negative voltage E will have been applied to the input terminals of the rectifier 38,V returningto zero value at the expiration of the delay. By reason of the characteristic of the rectifier, here assumed to bea full-wave rectiner, the signal delivered to the lamp 4l and therefore the illumination of the sensitive paper I' will be the same for a negative rectiiier input signal as it is for a positive rectier input signal. This rectifier 38 may be of any desired type, for example. a relay network adapted to deliver signals of one polarity when actuated either by positive or-by negative signals. If the Shaper, amplifier, and exposure lamp respond alike to positive and negative signals, the rectifier maybe dispensed with.

Referring again to-Fig..5, the curve Gshows the signal output from the relay circuit and rvthe signal input to'the'rectier 3 8. It rises toa value +B vupon the arrival Vof the signal and falls to zero after the speciiied time delay, then increases negatively to thevalue E at the rst negative transition of the signal curve E or curve F of Fig. 5 returning again to zero after. the expirationof the specified time delay. The curve kH represents the output of therectier, which is identical with the curve G except for a reversal Inspection will show that thiscurve J is identical with the curve A'which represented the light and daricvalues of the copy to be transmittedlexce'pt f for a time delay consisting of' two parts, iirsta delay deliberately introduced by the time delay device l1 inthe A'phase channel at the transmitter and substantially equal to the time required to scan a single dark area of thecopy; and al minor delay due to the sum totalof delays in the transmission line and vthe receiver apparatus. i

As is well known, vsuch delays are unimportant because they represent merely a displacement of the reconstituted copy in position on the drum and not a degradation of its quality.

.If desired, for example in order to attain greater precision .and temperature, independence inthe operation of the receiver apparatus time delay, an electrical network for producing this delay may be resorted yto' and the dash-pot35 "accable signal (curves E or F of Fig. 5) simultaneously energizes two relays 32' and 33 whose tongues therefore move from one fixed contact to the other substantially instantaneously. Upward movement of the tongue. of the lower relay 33 Yplaces a Avoltage E from the battery 31' on the lower input terminal of the rectifier 38 and downward movement of this tongue grounds this terminal. Upward movement of the tongue of the upper relay 32 applies a signal due to a battery 43 to the input terminals of an adjustable time delay network 44 and downward movement of this tongue removes this signal. The output terminals of the delay network 44 are connected to a third relay 45 of which the lower fixed contact is connected to ground, the upper fixed contact to the positive terminal of a battery 36', and the movable tongue to the upper input terminal of the rectifier.

In operation, as is evident from a glance at lthe circuit, the initial positive transition of the incoming signal (curve E' or F of Fig. 5) causes simultaneous operation of the firsttwo relays 32 and 33. Operation of the lower` relay 33I immediatelyplaces a voltage E on the lower terminal of the rectifier 38 and, since its upper terminal is grounded, causes illumination of the `lamp 4| as above described and exposure of the photographically sensitive paper I at a certain position. Simultaneous operation of the upper relay 32' energizes the delay network 44 and after the lapse of a preassigned time delay, the third relay 45 is energized. Its tongue then moves upward and ground is removed from the upper terminal of the rectifier 38 which is connected instead to the battery 36' thus'reducing the input voltage of `the rectifier` 38 to zero and extinguishing the lamp 4I. The two singletongue relays 32 and 33' may be replaced by a single two-tongue four-contact relay, like the relay I8 ofFig. l, if desired. l

As is well known, electrical and acoustic delay networks can be constructed to operate. with high precision and with constancy under various conditions of temperatureV and humidity. Thus theA extinguishing of the exposure lamp 4l may be caused to take effect at an instant which occurs with great exactness after a specified time delay commencing with the illumination of the lamp.

It will be understood by those skilled in the art that adjustment of the time delay produces a corresponding variation in the length of the reproduced dark areas only and therefore results in flexibility and variation, as distinguished 'from degradation, of the reproduction. For example light face typemay be 4reproduced as bold face or Clarendon type or vice versa, merely by ad justment of the delay time of the network 44.

Returning now to consideration of the appara- "l0 chronism, neither one adds substantially tothe information content of the signal derived from the other. Therefore, `and. at the sacrifice of a small amount of quality in the reproduced copy due to failure to distinguish between dark areas of standard length and dark areas which are vslightly longer or shorter than the `standard length, the apparatus may be greatly simplified and one or other member of the transmitter relay pair may be dispensed with. For example, in Fig. 1 the entire A phase equipment, including the time delay device l1 and the associated fourterminal two-tongue relay I8 may be dispensed with. Similarly in the -apparatus of Fig. 3the A phase equipment including the time delay device l1 and the associated relay lB" may be dispensed with. ,In each case this permits simplification of the phase splitter apparatus IB by omission of the `elementswhich control the currents of the A phase. In eithercase the remaining relay I9 or t9 may be arranged to be energized and deenergized successively upon successive occurrences of :a transition of a preassigned polarity in the curve B signal. That is to say, the remaining relay maybe arranged to produce as an outgoing signal on the transmission line 23 an intermediate sequence of signals which` are related to the B curve signals either in the ,mane ner of the Ccurve or in the manner of the D curve. `In either case the final outgoing signal will resemble, in wave form, the signal of therE curve at all points except `the nal portion` of the second signal group at which the rel-aysdid `not operate in synchronism. i v Thus, employing the characteristics of Vcopyof the preferred form to the best advantage results, not `only in a saving of signaling speed, Abut also in an economy of apparatus.

The apparatus of Figs. 1 and 3 may if desi-red be constructedto operate in either manner. With the switches 'Si or Saclosed, they operate as first described above. Opening the switches S1 orSs disables the A` phase equipment and operation takes place as last described above. i

Another modification of the invention, employing similar though not identical principles and utilizing a similarthough not lan identical sequence of operations, is illustrated for the transe mitter apparatus in Fig. 6 and for the receiver apparatus in Fig. 7, the sequence of signal cur-r rerit transformationsbetween the koriginal copy at the transmitter and its reproduction at the re-A ceiver being dia-grammatically illustrated in Fig. 8. For illustrative reasons, the apparatus 'of Figs. 6 and 7 is shown as being constructed of com-` ponents which lare mainly electronic in nature.

As will be `understood by those skilled in the art,

be employed or whether the simpler current -add- "ing larrangement of Fig. 3 be employed instead.

In either case, when both relays operate in synelectromechanical relays may be substituted as desired or as may be found convenient for these electronic components, just as electroniecomponentsmay in turn be substituted for theelectromechanical relaysof Figs. 1, 2, 3 [and 4.

Referring no-w in detail to Fig. 6,`the light source, scanning drum, photoelectric cell and `afs" sociated Vapparatus may be identical with-those of Fig. 1 andmay operate in a similar way to that hereinabove described, to produce at the section X-X of Figs. 1 and 6` an initial current of conventional for-m whose amplitude is proportional both in magnitude :and in duration to the su'ccessive dark and light areas of the copy to be transmitted. This relationship is illustrated in curves A and I0 of Fig. 8, wherein curve A represents the light and dark values of a part of a scanning line of the copy to be transmitted and thecurve Io represents the resulting image signal. This signal is now applied to the analyzing apparatus of Fig. 6 to produce signals for point to point transmission ywhose transitions occur much moreslowly 'than those of the curves A and Io.

VThus this [conventional image signal, designated Io, enters the primary winding of a repeating coil or transformer T1. The latter, by virtue of its well-known characteristic, transmits only negative or positive impulses of current corresponding, respectively, to the ligh-t-t-o-dark transitions and the dark-to-light transitions of the scanned copy. Two separate secondary circuits are connected to the secondary winding of the transformer T1 of which the rst includes rectifier Rc1, the input terminals of a delay device D1 and the primary winding of a transformer T2. The other secondary circuit of the transformer T1 includes the primary winding of a transformer T3 and 1a rectier Re2.

' The output of the delay device D1 is coupled ,by way of a transformer T4 to the control grid of a gas tube G1 whose cut-off condition may be adjusted by proper selection of its grid bias battery B2 in Well-known fashion. The anode of this gas tube G1 may be connected by way of the secondary winding of the transformer T3, a network N1 to be described hereinafter .and a source of anode voltage B1 to the cathode of another gas tube G2 and the negative terminal of a condenser C1. The positive terminal of the condenser C1 is connected to the anode of the gas tube G2 and also, by way of resistors R1 and R2, to the control grids of two vacuum tubes VT1 and VT2 whose cathodes may be connected 'together 5 tive pulse is applied simultaneously to both secondary circuits of the transformer T1. However, the rectifier Rez prevents the flow of current in the second circuit Vdue to this positive p-ulse,

whereas the rectifier Rc1 permits the flow of current I1 which, in turn, results in the application of a positive pulse to the control grid of the gas tube G2. lThis gas tube will then pass current in its anodecircuit if, and only if, the condenser C1 has theretofore been charged to a voltage of the polarity indicated by the plus and minus on the drawing, due to a previous signal. The ignition of the gas-tube G-z will thus operate to discharge the condenser C1 as long as the gas tube G1 is extinguished.

The application of the same positive impulse of I1 to the input terminals of the delay network D1 produces in the output circuit of this network a similar current impulse I2, slightly delayed with respect. to the current impulse I1. See curves I1 and 1210i Fig. 8. This delay is to be adjusted to a value vsufficient to permit the substantial discharge of condenser C1 and the subsequent extinction of the gas tube G2.` Upon the occurrence of the delayed positive pulse I2, the control grid of the gas tube G1 is raised above its cut-off voltage as determined by the potential of the anode battery B1 and the grid battery B2. Thereupon the gas tube G1 is ignited and current from the battery B1 flows through the network N1,

through the anode-cathode circuit 'of lthe gas tube G1 and thence to recharge the condenser C1.

In accordance with the invention this-recharging takes place preferably at a substantially-constant rate and endures for alcertain specified time'to be discussed hereinafter. The constancy of the charging rate may be achieved, as is wellknown in the art, by adjusting the relative magnitudes cf the parameters of the circuit so that the impedance oiered by the condenser C1 :to the electromotive force of the battery BrisV a comparatively small part of the impedance of the circuit as a whole. Thus, if the network N1 is merely a high resistance, or if the anode-cathode resistance of the gas tube G1 is high, this result will be attained, atleast over a comparatively short time, which may, in a practical case, be suflicient. If greater constanc;T of charging rate is desired than is obtainable with a simple linear resistor for the network N1, a more complex circuit arrangement may be employed.. With a simple resistor the condenser charge will accumulate along an exponential curve, as is well-known. The charge-time curve may be made substantially linear over considerable lengths of time by employing, for the network N1, a circuit arrangement including anon-linear resistorwhose'ren sistance varies in a prescribed mannerv in accordance with the voltage across it. Again, if desired, a feedback control circuit may be employed in which an auxiliary signal, directly proportional to the voltage on the condenser C1, is causedto control its charging rate in such a way thatit remains substantially constant over a considerable length of time. Any desired circuit arrangement may be employed for this purpose and, furthermore, itis to be regarded as a refinement and not a necessity because minor departures of the charging rate from constancy do not seriously degradethe quality of the reproduced copy.y

The charging rate of the condenser C11 is de-j termine-d by the parameters of the chargingcircuit. The charging commences at the instant at which the first positive pulse of I2 occurs and the gas tube G1 is ignited, and continues until'the `gas tube G1 is extinguished. This takes place when the anode ofthe gas tube G1 is driven below cut-off by a pulse or voltage in the secondary winding of the transformer T1, which is due tof'a negative current pulse in its primary winding; i. e., a pulse corresponding to a negative or blackto-white transition of the copy. This pulse' I3 occurs upon the arrival of the rst negative transition of the current Io, and flows through the second secondary circuit of the transformer T1 but is prevented by the rectifier Rc1 from aifecting any of the components of therst secondary circuit of the transformer T1.

The successive operations of the system as thus far described are graphically depicted in Fig. 8 wherein the curve A represents a typical portion of the copy being scanned having alternate light and dark areas. Unlike the system of Figs. l to 5, inclusive, there is no restriction on the dark areas that they be nearly alike in duration, and departure ci the copy from the yfornl'preferred for use with the embodiment'rst described does not produce degradation when the apparatus of Figs. 6 and 7 is employed.r

Curve lo of Fig. 8 shows the conventional (inverted) image signal derived from scanning the copy areas of curve A. The curve I1 shows the positive pulses of current which occur on positive transitions of the current Io and flow in the rst-named secondary circuit of the'transformer T1. The curve I2 shows the positive pulses derived from the positive pulses of curve I1 which are slightly delayed with respect thereto. The curve I3 shows the pulses which flow in the second-named secondary circuit of the transformer T1 which occur on the negative transitions of the current It, .corresponding to transitions from dark to light on the copy. The curve V1 shows the voltage on the condenser C1 which, as above stated, starts charging upon the occurrence of each pulse of I2, stops charging and holds its charge upon the occurrence of each pulse `of Is and is again discharged on the occurrenceof each pulse of I1.' Inspection of the curve V1 shows that in eachcase the magnitude of the charge is proportional to the length of the dark area of the copy whereas the duration of the charge is approximately equal to the durationof a copy area pair, as above defined, i. e., the duration of a dark area and the following light area.

This sequence of intermediate signals V1, could, if preferred, beA transmitted to the receiver station without alteration ofform. However, it is preferred to reverse the polarity ofY alternate signals of this sequence as shownby the curve V2 in Fig. 8 prior to transmission in order that it maybe less subject to degradation in the course Aof transmission.l The reason is that a transmission channel whoserange of transmitted frequencies is limited as, for example, is the case l' with a composited telegraph circuit, would transmit the slowly alternating voltage V2 without excessive distortion but would not pass the interruptions in the direct wave V1, because they occur at twice the' rate of thosefof the Wave V2. To this end, the circuit arrangement shown at the right-hand part of Fig. 6 is employed and of which a detailed description was earlier postponed. This circuit comprises `two main portions, in particular a first or signal path portion comprising two vacuum tubes VT1 and V'Iz having resistors R3, R4, Rs and Re connected in their anode" circuits in the manner shown, and resistors R1 land R2 connected in their grid cir-` cuits.j The second main portion of this circuit 'arrangement `is a feedback network which applies to the control grid of each of the tubes, for example, VTz, aportion of a voltage derived from the output current of the other tube, for example,

VT1, in a manner to paralyze one tube while the other is acting as an amplifier, and to change the conditions of paralysis andactivity upon the occurrence of each new signal ofthe sequence; il4 e., each time the voltageV1 drops to zero.

The detailed description of the operation of this feedback network willbe againpostponed, and the operation of the main signal path portion ofthe `circuit will be first described on the assumption that the feedbackl circuit" operates asintended. Thus,.with this circuitarrangement, only one of the tubes VT1'v onVTe' can be active at a'time, and it remains active, despite the presence of a signal applied to the control grid ofthe other tube, until after the first tube has been deactivated and paralyzedby the removal of thesignal applied to its control grid. Thus, let it be assumed that the tube V'Iz has been deactivated. The rst rise of the voltage V1 of the condenser C1 is applied to the control grids of both tubes in parallel, but only the tube VTi delivers an output current. The amplified output voltage which appears across the resistors Re, R5 and therefore on the output transmission line 23, `will be substantially proportional to the voltageapplied to the grid circuit of this tube,

namely,y to the voltage V1. This is indicated in thecurve V2. Howeveryupon discharge of the condenser C1 a pulse is fed back fromthe resistor Re in the anode circuit ofthe tube VT1 tothe resistor R2 and appears as a positive voltage pulse on the grid of the tube VT-z. This places" this tube in active condition, and a small Surgof current passes through it. Thisis sufficient to send a .feedback voltage, derived from thelresistor R4 to the resistor R1 to render the tube `VT1 inactive, so that, as the condenser C1 is recharged with the next voltage signal of the curve V1, it is the tube VTe which amplifles this next signal while the tube VT1 remains inactive. Witnth'e output resistors R5 and Re connected, as shown, there results a voltageV on the transmissionline 23 whose signal cycles are alternately inverted with respect to the condenser voltage V1. VThis alternating inversion of the voltage V1 is illustrates in the curve v2. f4;

Coming now to a description of the feedback circuit, a transformer Te is provided, its` primary winding being connected by way of a resistor `Ri to the terminals of the resistor Re in the anode circuit of the tube VT1 and its secondary winding being connected by way of a resistor Rs to the terminals of the resistor R2 in the grid circuitof the vacuum tube VTz. The windings, should be so poled that a positive voltage input to the trans# former Ts, for example, corresponding, to the rising portion of the curve V1 of Fig. `8,.applie's a negative voltage to the grid oftheltube in opposition to any positive voltage on con,- denser C1. During an increase in the voltage V1, and' assuming that the amplifier `tube VT1 Yis active, its anode currentvexhibits a substantially constant rate of increase. This anode current flows through the resistor Ra and the primary winding of the transformer Ts in parallel. With proper apportionment of the impedances ofthese elements, the primary winding current increases at a substantially constant rate.V Thus, there is generated in the secondarywinding of this transformer, and therefore applied to the .controlgrid ofthe tube VTz, a substantially constant electro- Inotive force. The transformer windings `should be so connected to the terminals ofthe resistorrRz that this voltage is negative, and the turns ratio of the transformerTe should be such that-the magnitude of this voltage is sufficient to hold the tube VTz below its cut-off point despite the application of the'voltage of the signal V1 toits control grid. V t

A high frequency oscillator O1 is provided, which supplies higlifreq'uency alternating current byway of a conventional hybrid coilI Vi-I. provided with a terminalnetwork Z, to the` grid-- cathode circuit of an amplier tube VTL; The.

. grid-cathode circuit includes a source Be of steady` bias, sufcientto hold the tube VT3 non-,conductive tothe oscillationsof the source O1 4in the absence of a signal V1. The resistor Reis also included in theV circuit and the" voltage; drop across R3 due to the plate currentinfVT1hasv such a polarity that when Athe output current: of the tube VT1 reaches a fair value, the biasingy effect of the source Bs is overcome, and thetube. VTi becomes conductive to the oscillations.: t The anode of the tube VTa is coupled Eby way of a transformer Ta, to a full wave .rectifier YRega. low pass filter F1 and a resistor R11 to the ter' minals of the resistor R2 in the grid circuit Vof the tube VTz. The polarity of the rectifier Riet' is so chosen that the resulting voltage applied" to ,the grid of the tube VTz, derived froinxthm amplification and rectification ofthe output of the oscillator-01, is negative. This negative voltage supplements that derived directly from the output ofthe tube'VT1 and delivered by Way of the transformer Ts to the grid of the tube VTz, during the period of the increasing of the' voltage V1.v Y

When'zthe voltage V1 ceases to rise, and remains constant asabove described at a value proportional to the yduration ofr the darli area scanned, the feedback voltage derived fromv the transformer Te is no longer effective to block the tube VTz, but the voltage derived from amplification and rectification of the oscillator output-continues to be as effective as before. Thus the .tube VTz remains paralyzed, .despite the application ofk signal voltages to its grid, while the tube VVT1 amplifies the signal voltage V1-and an `amplified `replica thereof appears on the transmission line 23, until such time as the voltage V1 falls to zero, at the conclusion of-the first full signal of this sequence, as hereinabove described. Thereupon the anode current Vof the tube VT1 ceases to flow, sov thatthe portion of the grid v.bias voltage of the tube VT1 which was derived from the anode current voltage drop in the resistor R3 ceases to exist. The voltage of the bias source Be now becomes controlling so that the tube VTa no longer passes signals from the oscillator O1. Thus the paralyzing signals ,fare removed from the control grid of the tube VTz. A t the same time, the abrupt fall of the signalV voltage V1 to zero andthe consequent sudden decay of the anode current of the tube VT1 produces a pulse of reverse polarity through the transformer T6, thus delivering a pulse of positive voltage to the grid of the tube VTi. rlhis .tube is now able to receive the voltage of ther ensuing signal of the sequence V1 and amplify it, in the same fashion that the .tube VT1 amplified the first signal. A feedback network which is similar to the one just described isassociated with the tube VT2, for supplying a paralyzing bias and an activating pulse to the tube VT1.

As soon as this tube VTz becomes active, a porv tion of its output current, flowing through the primary winding of a transformer T7, delivers a substantially constant negative bias voltagev to the resistor R1 and so to the grid of the tube VT1. At the Ysame time the voltage of the oscillator O1 is delivered Yby way of the hybrid coil H to the grid of the amplifier VT4, and the signal voltage across the resistor R1 is likewise applied to the control grid of the tube VT4 to overcome the bias of the battery B1. As a result, the tube VT4 ampliiies the oscillator output. This is applied by Way of the transformer T9, the rectifier Ra, the filter F2 and the resistor R12 to the grid circuit of the tube VT1 to hold it negative Yand so maintain the paralysis of 'this tube until the anode current of the tube VTz shall abruptly fall to zero at the conclusion of the second signal of the sequence V1. Thereupon the voltages applied to the grid of the tube VT1 from the transformers T'rand Ts cease to be negative and, instead, a positive pulse is applied to the grid of the tube VT1 for a short interval, whereupon control,

16 clude the primary Winding of the transformer Ts, the resistor R1, Ythe resistor R5, and the potential source Bs, the anode-cathode current path of the'tube VTz being similar.

The' resistors R'z'and R9 are included merely to prevent the primary windings of the transformers Ts .and T1, respectively, from short-circuiting direct voltages whichv appear across ythe resistors Ra land R4. The vinherent resistance of the primary windings of the transformers Ts and T7 may Well be suiiicient for this purpose.

The resistors R8 and R11 constitute a combining pad to prevent short-circuiting of the output of the rectifierRer by the secondary winding of the transformer Ts. These resistors also serve to partially isolate the filter F1 from theY circuit of the postivepulse which occurs when the voltage- V1 abruptly falls to zero, so that this pulse shall not be too greatly changed bythe reactance of the low-pass filter. The same remarks apply to the resistors R10 and R12, with respect to the transformer T7, the rectifier Res, and the filter Eb.

In Fig. 6, each of the gas tubes and vacuum tubes is-shown as being provided with its own grid bias `battery and its own plate and filament supply batteries, as well Vas its own input circuit. It is, of course, possible to make Wide variations in these `details in accordance with well-known practice. Heater-cathode tubes may be employed as desired. n s

A switch Ss is provided for grounding the midpoint of the inverting amplifier at the transmitter, if desired. This tends to balance the line 23 to ground and therefore to reduce the amount of pick-up of stray voltages and the yconsequent noise. Many other modifications maybe made to effect economies of apparatus or improve the characteristics of the system without departing from the principles of the invention. Y

Fig. 7 Y shows receiver apparatus adapted to decipher' the received signals (curves V1, V2' of Fig. 8) and to reproduce thetransmitted copy therefrom. This figure will be explained in terms of .the received line signal sequence having the form of the curve V2, it being understood that, if on receipt of the signals at the receiver station, they are .seriously rounded, they may be Squared up in accordance with well-known techniques or if the .distortion is not serious they may be utilized as received, the result beingthe same as though they had the form of the signals of the curve V2 but very slightly delayed with respect thereto.

Referring now in detail to Fig. '7, the incoming signals may be conceived of as arriving ,onv the transmisison line 30 where, after such amplifica- VVin the remainder of the circuit to be described Vshall not substantially modify the rectifier current output. Y

'The primary winding of a transformer Tn is connected across the resistor R13, the secondary winding being connected 'to the control grid and by way of a biasing battery Ba to the cathode of a gas discharge device Ge. The anode of the gas tubeGs is connected to one (negative) termiacoegeie 'nal of a condenser C; and tothe cathode of another gas tube Gs. l

`The positive terminal of the condenser C3 1s connected by way of a resistor R tothe control grid ofa gas tube G1 whose cathode is returned, through the resistor R14 and a grid bias battery Be to the negative terminal of the condenser C3. The anode of this tube is connected by way of the primary winding of a transformer T10 and an oscillator O2 to its cathode. This transformer T10 is provided with two similar secondary windings, one of which is connected by Way of a bias battery tothe cathode and control grid of a gas `tube Gs and the other of which is similarly connected by way of a bias `battery to the cathode and control grid of another gas tube Cia.` The anodes of these two gas tubes G9 and Gs are connected together and to the positive terminal of the condenser Ca. Thecathode of the gas tube Gg is connected by Way of an oscillator Oa and thence by way of a resistor Ri to its anode and to the positive terminal of` the condenser C3. This cathode is also connected by way ofthe oscillator Os, a load impedance 61 and a source of operating potential Bio. to the cathode of the tube Gs.

VThe load impedance 61 may be of any type suitable for copy reproduction. For example, it may comprise the moving element of an oscillograph here shownas comprising a source of light 65, a

`vibratory mirror 66, the actuating coilSl, a lens -Gll by whichlight rays from the source E5 after reflection by the mirror 66 are` brought t o a focus on a Vreceiver drum 2', which may be similar in .all respects to the receiver drum of Fig. 2 and may .be-similarly, driven. VBetween the vibratory mirror 66, and the lens B8 there may be placed `alight intercepting screen 69 having an opening `'lll therein through which the light rays may pass when the element 61 is energized, the screen 69 cutting them off when the element 61 is not energized.`

It will be understood by those skilled in the art that the various operating potential supplies, indicated on the drawing as batteries, may be provided by any suitable means, that cathodes which are independent of the cathode heating elements may be employed instead of the fllamentary cathodes shown, that tubes may be employed having various auxiliary grids, properly supplied with potential, and other similar modificationsin the details of. the` circuit may be employed without in any way modifying its operation as hereinafter described. v

In operation, as a signal sequence, depicted as the curve V2 of Fig. 8, arrives on the incoming transmission line 30, it is restored by the rectiiier Res into the signal sequence form represented by the curve V1, and appears as a voltage drop acrossthe resistors R13 and R14. The characteristics of the transformer To are adjusted in Wellknown manner so that the voltage which appears across the output terminals of its secondary winding has a wave form consisting of a sharp positive pulse each time the rectified signal (V1) falls to zero, followed bya substantially constant .negative value during thetime throughoutwhich the signal rises'at al constant rate,`as above described. This wave, form isrepresented in Fig.

,8 as V4.

The initial surge of thefvoltage V4 places a momentary positivevvoltage peak on the grid of the gas tubeGs. The biasing source Bs is so adjusted that any positive voltage acting between 'the cathode and the grid will cause the cathodeanode path of the tube Ge to become conductive. Therefore, whenever the current incoming on the `current charges the lcondenser C3 with 'the polarity indicated in the figure. The condenser is preferably of large capacitance, in order that the voltage to which it is charged shall never become more than a small fraction of the total voltage drop in the cathode-anode circuit of the tube G6. This precaution assists in maintaining constancy of the charging current and so in charging the condenser C3 at a substantially constant rate. The substantially constant charging rate of the condenser C3 results in a condenser voltage E5 (Fig. 8) which increases positively in magnitude at substantially the same constant rate. This voltage is impressed, through the resistor R15. on the control grid of the tube Gv in opposite phase to the voltage V1 appearing across the resistor R14. Thus the voltage impressed on the control grid of the tube G7 is in effect the difference between the voltages E5 and V1. By proper selection of the values of the anode source B10 and the resistors R13, R14 and R15 these two voltages may be made substantially equal in magnitude during the period of the condenser charge. As `above stated, they are opposite in sign. Therefore, during this time, the voltage impressed on the grid of the tube Gv due to the received signal is substantially zero.

At a time corresponding to the time at which the Voltage V1 at the transmitter ceases to increase and becomes constant, the tube Ge is extinguished in the manner described below. However, this doesnot occuruntil the apparatus yet to be described has had time to act, and therefore the voltage impressed on` the tube G7 from the resistor'` R14 which is directly related to the received voltage V1 ceases to rise and becomes fiat before the voltage E5 ceases to rise. Therefore, at the instant at which the voltage V1 ceases to rise and become fiat, the control grid of the tube Gv is driven to a positive value by the continuing increase of the condenser voltage E5. The biasing source B9 and the amplitude of the voltage of theoscillator O2 are so adjusted that a very small increase in the control grid voltage will cause the tube G7 to fire. This occurs on a positive half cycle of the voltage of the oscillator O2 and sends a sharp surge of current through the primary winding of the transformer T10, thus inducing voltages and in the secondary windings of this transformer Vand impressing these voltages on the control grids of tubes Gc and Gs. i

The positive control grid voltage pulse on the tube G9 causes momentary ignition of this tube during a positive half` cycle of the voltage of the oscillatorOa. The next half cycleof this oscillator causes the .tube G9 t0'.be1extinguished.` By the time the `following positive half cycle occurs, .the grid voltage surgerh'as ceased so that the tube G9 is ignited only for one-half cycle of the oscillatorvoltage. VThis momentary ignition of the tube 'causes asharpsurge of current to flow in the cathode-anode' circuit ofthis tube which includes the resistor R1ewhich, as above stated, vhas a `high value. ',Thevalu'es. of `thisresistor R16 and of the voltage of the oscillator O3 are so adjusted that duringits brief current surge the voltage across the resistor R16 'outweighs'the voltage of the anode source B so that, the voltage on the grid of the tube G6 being no longer positive, the ktube Ge is extinguished.` Extinction of this tube causescharging of the condenser C3 to cease and, as above stated, the condenser voltage now remains momentarily at a constant value which is proportional to the length ci time during which the condenser C3 was being charged. This, in turn, as described above in conjunction with Fig. 5, is proportional -to the duration of a dark area of the transmitted copy.

The positive surge in the grid circuit of the tube Garenders the cathode-anode circuit of this tube conductive. rIhe only source oi anode voltage in the anode circuitA of this tube, when tube Gs is extinguished, is the charged condenser Cs. Therefore this condenser rapidly and immediately discharges throughV the cathode-anode circuit of the tube Gs, thus reducing the control grid voltage on the tube G7 past cut-off vso that,

on the next half cycle ofthe voltage of the oscillator O2, this tube is not reignited. The Wave form of the control grid voltage of tube G7 is shown on Fig. 8 as Ee.

As a, consequence cf the actions above described, the voltage E5 of condenser C3 rises at a substantially constant rate for a time very slightly greater than the time during which the received signal V1 or V2 increases, positively or negatively, ata constant rate. At the conclusion or this time the voltage suddenly drops to zero; As above described, during the charging period, the charging current Ia ilovvs in the cathode-anode circuit of the tuheGe and through the sensitive element 61, causing deflection of the mirror 66 to a position such that light from the lamp G5 passes through the aperture 1li and exposes the film I. At the conclusion of the charging time, the light beam is deflected to a position at which the screen 68 interceptsjit. Thus exposure takes place during successive times which are substantially proportional to Vthe magnitudes of the pulses of the incoming signal sequence, and independent of their polarity. This results in eX- posure of the photographically sensitive film or paper in a manner to reproduce the copy being transmitted, as indicated in curve J of Fig. 8.V

Various modications, both of components and yof their arrangement and organization, may be made Without departing from the inventive spirit and principle of utilizing characteristics of the copy to be transmitted in order to reduce, without serious degradation of the reproduction, the signaling speed, and therefore to effect economy eitherl of transmission apparatus or of transmission time.

What is claimed is:

l. In an image signal transmission f system, means for scanning successive line series of elemental areas of tWo-tone copy to produce a se-v quence of image signals, each signal of said sequence corresponding to a scanned area of onetone value, means for deriving from said sequence an intermediate signal sequence, each signal corresponding to a pair of adjacent copy areas of unlike tone values, means for transmitting said intermediate signal sequence to a receiver station, and, at said receiver station, means responsive to each transition of said intermediate signal sequence for initiating a reproduction signal pulse, means for thereafter terinating said pulse prior to the inception of the ensuing pulse, said pulses constituting a reproduction signal sequence, image reproducing means. and means for `actuating said reproducing means by said reproduction signal sequence.

, 2. In animage'fsignal transmission system. means for scanning successive line series of elemental areas o f two-tone copy Vto produce a sequence of image signals, each signal of said sequence corresponding to a single scanned area, means for deriving from said sequence ari-intermediate signal sequencaeach signal corresponding to a pair of adjacent copy areas of unlike tone values and having ,a duration equal to the duration of the image signals derived from said pair, means for transmitting' said intermediate signal sequence to a receiver station, and, at said receiver station, means responsive t9 each transition of said intermediate signal sequence for initiating a reproduction signal pulse, means for thereafter terminating said, pulse ,prior to the inception ofthe ensuing pulse, `Suid .pulses constituting a reproduction signal sequence. image reproducing means, and means for actuating said reproducingfmeans .by said reproduction signal sequence. Y

3. In an image .signal .transmission system, means for scanning" successive ,lineA series of elementalV areas `ofl vtwo-tone Vcopy toV produce a sequence of image signals, each signalof said sequence corresponding-to a single scanned area, means forV deriving/from said sequence an Yintermediate signal sequenca'each signal corresponding to a pair-of adjacentcopylareas ofunlike tone values and -havinga duration equal to the duration ofthe image signals derived` from said pair and vhaving ranother characteristic related to the duration ci -a particular member of said pair, means for transmitting said Aintermediate signal sequence k-to a receiver station, and, `at said receiver station, means responsive toeachtransition of said intermediate signal sequence for initiating a reproduction signal pulse, means for thereafter terminating said pulse prior to the inception of the ensuing pulse, said pulses constituting a reproduction sig-nal sequence-image reproducing means, and means for actuating said reproducing means Yby said reproduction signal sequence.;

4i. In an image signal transmission s-ystem, means for scanning successive line series of elemental Vareas of two-tone copy to produce la sequence of image signalseach signal of said sequence corresponding toy a single scanned area, means for deriving from said sequence an intermediate signal sequence, each signalV corresponding to a pair of adjacent copy-areas ofg unlike tone values and having a duration equalto the duration of the imagev signals derived from said pair and having another characteristic related to the duration of a particular' member of said pair, and means `for transmitting said intermediate signal sequenceV to ajrece'iver'station for Areproduction thereat of said copy.v Y

5. In an image signal; transmission system, means for scanning successive line series of elemental .areas of two-,tone cQDy t0 produce a sequence of image signals, each signaly of said sequence corresponding 'to a single scanned area, means for deriving from said sequence an intermediate signalsequence, eachsignal corresponding to a pair of adjacent copy areas of unlike tone values, and means 'for transmitting said intermediate signal sequence to a receiver station for reproduction thereat of said copy'.

6. In a system for deriving facsimile signals from black and. White-copy.ap1ura1itv of distinct Communication channels means far producing in one of said channelsa first sequence of signals having transitions corresponding only to points of the copy `at'vvhich black follows white, means for l producing in` another of said channels a second `sequence of signals having transitions corresponding only to points of the copy at which white follows black, means for delaying one of said signal sequences to bring its transitions into approximate coincidence with the transitions of said other sequence, and means `for combining the signals of said delayed sequence with the signals of said undelayed sequence to form a composite signal `suitable for economical transmission.

7. Apparatus as defined in claim 6 wherein said signal-combining means comprises means for arithmetically adding the instantaneous values of said delayed signal to the instantaneous values of said undelayed signal.

8. Apparatus as defined in claim 6, wherein said signal combining means comprises a circuit arrangement Which is similarly responsive `to the presence of a signal of either of said sequences and doubly responsive to the simultaneous presence of signals of both of said sequences.

9. Apparatus as defined in claim 6 wherein said signal combining means comprises a sourceof voltage, terminal apparatus, means for applying the voltage of said source to said terminal apparatus, means responsive to the presence of a signal of one of said sequences for reversing the polarity of the voltage applied to said apparatus Without affecting its magnitude and means responsive to the presence of a signal of the other of said sequences for altering the magnitude of the voltage applied to said apparatus without affecting its polarity.

10. In a system for reconstituting a replica of two-tone copy scanned at a transmitter station from a received sequence of signals each of which corresponds to apair of adjacent scanned copy areas of unlike tone values, means responsive to each transition of said received signal sequence for initiating a reproduction signal pulse of preassigned polarity, means for thereafter terminating said pulse prior to the inception of the ensuing pulse, said pulses constituting a reproduction signal sequence, image reproducing means, and means for actuating said reproducingmeans by said reproduction signal sequence.

`1I. In a system for reconstituting a replica of two-tone copy scanned at a transmitter station froma received sequence of signals, each of which corresponds to a pair of adjacent scanned copy areas of unlike tone values, means responsive to each transition of said received signal sequence for initiating a reproduction signal pulse of preassigned polarity, means for terminating said pulse after the lapse of a preassigned time interval, said interval being substantially equal to the time required at the transmitter station to scan a signal dark area of standard length, said pulses constituting a reproduction signal sequence, image reproducing means, and means for actuating said reproducing means by said reproduction signal sequence.

12. In a system for reconstituting a replica of two-tone copy scanned at a transmitter station from a received sequence of signals, alternating in polarity, each of said signals corresponding to Ia pairof adjacent scanned copy areas of unlike tone values, means responsive to each transition of said received signal sequence for initiating a reproduction signal pulse of preassigned polarity,

the polarity of @said pulse beingindepen'dent of the polarity of said transition, means for thereafter terminating said pulse prior to the incep-V tion of the ensuing pulse, said pulses constituting a reproduction signal sequence, image reproducing means, and means for actuating saidreproducing means by said reproduction signal sequence.`

13. In an image signal transmission system, a field to be transmitted comprising an arrangement of areas of one tone value separated by areas of anotherV tone value, means for producing a sequence of signals of alternating polarity, each of said signals having an amplitude related to the tone value of an area of said field and a duration related to the combined lengths of said area and an adjacent area of the other tone value, means for transmitting said signals to a receiver station, and at said: receiver station means for reconstituting a replica of said iield from said signals. f'

14. In a system for transmitting facsimile signals from two-tone copy, a condenser, means for discharging said condenser at an instant corresponding to a point of said copy at which an area of a rst tone value follows an area of a second tone value, means for thereafter immediately initiating the' charging of said condenser, means for causing said charging to proceed at a substantially constant rate for a time corresponding to the length of said area of the iirst tone value, means for terminating said charging at an instant corresponding to a point at which an area of the second tone value follows said area of the first tone value, means for holding the charge for a time corresponding tothe length of said area of the second tone value, means for repeating said discharge, charging and charge-holding for each copy area pair, and means for deriving from the resulting varying condenser charge a signal suitable for transmission.

15. In an image signal transmission system, means for scanning successive line series of elemental areas of two-tone` copy to produce a sequence of image signals, each signal' of said sequence corresponding to the scanning of an. area of one tone resulting in adjacent signals being of different tones, and means for` deriving from said sequence an intermediate sequence of areapair .signals for transmission, each signal of said intermediate sequence comprising a portion of uniform slope corresponding to the rst area of said pair and a portion ofuniform height corresponding to the second area of said pair.

16. In apparatus for reconstituting a replica of two-tone copy means producing a sequence of area pair signals, each signal of said sequence having a portionof uniform slope corresponding to onearea of a copy area pair and a portion of uniform height corresponding to the second area of said pair, a condenser, means for charging said condenser through one current path at a substantially constant rate throughout the persistence of the uniform slope portion of each signal of said sequence received from said source, means for thereafter discharging said condenser through another `current path, and a current-sensitive image-reproducing element in said first current path. y

17. Apparatus as defined in claim 14: wherein said transmission signal deriving means comprises means for producing a voltage of one polarity proportional to one condenser charge of said sequence and` for` producing a voltage of opposite polarity proportional to the ensuing condenser charge of said sequence. fr

i8. The method of.-v two-tone copy facsimile transmissionwhich coniprise's scanning said copy to derive an image sigr-i'al sequence, deriving from said image signal 'sequence a sequence ofi intermediate signals, each of said signals 'commencingwith the start'of scanning an area'of one tone value and terminating with the start of scanning of thenext area' of said tone value, transmitting said intermediate signal sequence to a receiver station, producing 'a sequence of reproduction signal pulses, cachot which commences With the transition of'said intermediate signals, terminating each ofsaid pulses after the lapse of .a xed time interval, rand reproducing an image in proportion to the signal values of said reproduction pulse sequence.-

19. Thelmethod of deriving transmission signals vfrom vscanned tWof-tonec0133 which comprises producing in one channel a iirst sequence oi signals having 'transitions corresponding only to points' of the. copy at Which-black follows White, producing in anotherof said channels a second sequence ,of signals having transitions corresponding only topoints of the copy at which whitev follows black, delaying one of said signal sequences to bring its transitions into approximate coincidence withV the transitions of said other sequence, and combining the signals Vof said delayed sequence with Vthe signals of said undelayed sequence to form a composite transmission signal. p l

20. in the vart ofreconstituting two-tone copy copy scanned ata transmitter station from a received sequence of signals eachy of .which correspends to a pair of adjacent scanned copy areasoi unlike tone values, the method Which'comprises initiating a reproduction signal pulse of preassigned polarity upon the occurrence of each transition of said received signal sequence, terminating each of said pulses'after the lapse of a fixed time interval and priorA to the inception of Vthe ensuing pulse, :and reproducing an image whose toney values are proportional tothe magnitudes of said'pulses.

21.-.Signal translating apparatus adapted to reverse the polarityof valternate signals of a received signal sequence and. translate thernvvithout other change of'character while translating intervening signals of saidsequencer with no change in character which comprises `a pair' oi translatingrdevices having input terminals and output terminals. .the input terminals of each ,of said devices including al control electrode, an input signal sequence source, the input terminals of both of said devices being connected in parallel to saidsource, a `translated signal utilization circuit, said output terminals being connected to said utilization circuit in push-pull arrangement, a feedback network interconnecting an output terminal of each of said devices with the control electrode of the other of said devices, said netv/'orl-:icomprising means responsive to the presence of current-,in the output circuit of each of said devices forholding the other of `said devices in. a state cf inactivity, thus paralyzin'geach of said devices-While the other istranslating the current of alsignal'oflthe sequence, and means responsive to an abrupt decay of the translated cirrent of each 'of said devices'for applying an activating voltage to the control electrode of the vother of saidV devices upon the completion of said signal, thus placing it in 'condition to translate the ensuing signal and to initiate paralysis of the first-named device. '-f' 22. Signal translating apparatus adapted to reverse the polarity of alternate signals of a received signal sequence and translate them Without other change of character While ,translating intervening Isignals of "said sequence with no change in character Which-comprises a pair of translating devices having input terminals and output terminals, the input terminals of each of said devices including a control electrode, an input signal sequence source, the input terminals of both of said devices being connected in parallel to said source, a translated signal utilization circuit, said output terminals being connected to said utilization circuit in push-pull arrangement, a feedback network interconnecting an output terminal of each of Vsaid devices with the control electrode of the other of said devices, said network comprising means responsive to the presence of current in the output circuit of each of said devices forholding the other of said kdevices in a state of'inactivity, thus paralyzing each of said devices while'the other is 'translating' the current ofv a signal of the sequencaand means responsive tok the terminationv of theY signal Vcurrent of each of said devices for applying an activating voltageV to Vthe control electrode of the other of ls aid devices upon the completion of said signal, thusy placing it in condition to translate the ensuing'signal and to initiate paralysis of the inst-named-device. l s

23. Signal translating apparatus adaptedto reverse the pOlarity of alternate signals of a received signal sequence and translate them Without other changeof character While translating intervening signals of said sequence with no change of character which comprises a source of a signal sequence, a pair of space discharge devices having their cathode-control electrode circuits connectedv in parallel and `their cathodeanode circuits connected in push-pull arrangement, a signal input circuit for applying signals from said source to the control electrodes of said devices simultaneously, asignal output circuit comprising the anodes of'botn of saididevices, and a feedback path from the Vvanode circuit of each of said devices to thercontrol electrode of the other of said devices, said path comprising means responsive tothe presence of current in the anode circuit of each of saidr devices for biasing the other of said devices past its cut-off point, thus paralyzing each of said devices While the other 1s conducting the current of a signal of the sequence, and means responsive to an abrupt decay ofthe anode current of each of saiddevices for applying a positive voltage surge to the control electrode of the' other of said devices upon .the completion of said signal pulse, thus rendering itconductive to the current of theensuing signal of the sequence and initiating paralysis of the first-named device. Y

24. Signal translatingv apparatus adapted tov reverse the polarity of alternate signals of a received signal sequence and translate them Without other change of vcharacter While translating intervening signals 'of said sequence with' no change ofv character which comprisesa source of a' signal sequence, a pair of space discharge devices having theirV cathode-control electrode circuits'connected in parallel and their cathodeanode circuits connected in push-pull arrangement, a signal input circuit for applying signals from sa1d source to the control electrodes of said devices simultaneously, a signal output circuit comprising the anodes of both of said devices, and afeedbackpath froml the 'anode circuit of each of'said devices to the control electrode of the other of said devices, said path comprising means 'responsive to the presence of current in `the anode circuit of each of said devices for biasing the other of said devices past its cut-off point, thus paralyzing each of said devices while the other is conducting the current of a signal of the sequence, and means responsive to the termination of the signal anode current of each of said devices for applying a positive voltage surge to the control electrode of the other of said devices ,upon the completion of said signal, thus rendering it conductive to the current of the ensuing pulse signal of the sequence and initiating paralysis of the rst-named device.

KENNETH W. PFLEGER.

REFERENCES CITED UNITED STATES PATENTS Name Date Armstutz Mar. 5, 1912 Number Number Name Date 1,627,701 Hall May 10, 1927 1,809,664 Angel June 9, 1931 1,826,970 Walker Oct. 13, 1931 1,848,839 Ranger Mar.l 8, 1932 1,910,556 McFarland May 23, 1933 2,175,388 Gurley Oct. 10, 1939 2,232,190 Vance -1 Feb. 18, 1941 2,281,891 Terry May 5, 1942 2,321,611 Moynihan June 15, 1943 2,333,245 Hansell Nov. 2, 1943 v2,349,810 Cook May 30, 1944 2,408,117 Young Sept. 24, 1946 2,430,547 Anderson Nov. 11, 1947 2,433,407 Talion Dec. 30, 1947 2,446,635 Cooley Aug'. 10, 1948 2,485,310 Patremio --..1 Oct. 18, 1949 2,488,314 Moore Nov. 15, 1949 OTHER REFERENCES Electrical Experimenten December 1917, pages 516 and 517. 

